A brake booster includes a valve mechanism which is controlled to open or close in response to the depression of a brake pedal, the movement of which is transmitted through an operating rod. The valve mechanism includes a variable pressure chamber and a negative pressure chamber. When a braking action is desired, the atmospheric pressure is introduced into the variable pressure chamber, and a pressure differential across the variable pressure chamber and the negative pressure chamber acts upon a power piston to achieve a booster function. On the contrary, when a braking action is to be released, the communication between the variable pressure chamber and the atmosphere is interrupted while a communication is established between the variable pressure chamber and the negative pressure chamber, allowing the power piston to be returned to its non-operative position under the resilience of a return spring. The fluid pressure applied to the power piston is controlled by the valve mechanism which is defined by a valve plunger. To prevent the valve plunger from being withdrawn from a valve body, the latter is formed with a radial hole, into which a stop member is inserted for engagement with the valve plunger. In addition, to prevent the disengagement of the stop member from the hole, various means are employed such as supporting the outer end of the stop member by means of a diaphragm. However, means in the prior art practice which is used to prevent the disengagement of the stop member has been less than satisfactory as a result of impending the sealing function of the diaphragm, producing strange sounds during oscillation or the inability to securely prevent the disengagement.